Attempts to create a cardiac muscle cell line which actively proliferates in culture while maintaining the differentiated phenotype have to date not been successful. Transgenic mouse lines have been derived in which the simian virus SV40 large T antigen expression has been targeted to atrial and ventricular cardiomyocytes of the heart [Field, Science 239:10029-10033 (1988); Katz, et al. Am J. Physiol 262:H1867-H1876 (1992)]. Expression of this oncogene under control of the atrial natriuretic factor (ANF) promoter results in atrial hyperplasia whereas expression of this oncogene under control of the .alpha.-cardiac myosin heavy chain promoter results in hyperplastic growth of both atrial and ventricles. AT-1 cells are atrial cardiomyocytes that were derived from the ANF-SV40 large T antigen construct [Delcarpio, et al. Circ. Res. 69:1591-1600 (1991)]. These highly differential myocytes have been maintained for over 6 years by serial passage as ectopic grafts in syngeneic mice. AT-1 cells can be easily cultured and maintain their cardiac phenotype; however, they can not be passaged in culture and must be maintained as a subcutaneous tumor lineage in syngeneic mice. On the other hand, a cardiac muscle cells line named AT-2 cells has been derived from atrial tissue of transgenic mice expressing SV40 large T antigen under control of the .alpha.-cardiac myosin heavy chain promoter. At-2 cells rapidly divide in culture (average population doubling time of 24 hours) and can be easily passaged, but they totally lose their phenotype after about the tenth passage in culture [Katz, et al. Am J. Physiol. 262:H1867-H1876 (1992); Borisov and Claycomb, Annals of N.Y. Acad. Sci., 752:80-91 (1995)]. Attempts by others to produce a cardiac myocyte cell line have resulted in a similiar pattern of loss of phenotype. One of the first reports of a cardiac myocyte cell line was on the isolation of a clonal cell line from rat heart [Kimes and Brandt, Exp. Cell Res 98:367-381 (1976)]. These cells fused to form typical skeletal myotubes and they expressed skeletal muscle-specific creatine phosphokinase. Contamination of these cardiomyocyte cell line cultures with skeletal myoblasts was not, however, rigorously excluded. v-myc transformed quail cardiac myoblasts have been passaged over 60 times [Jaffredo, et al. Exp. Cell Res. 192:481-491 (1991)]. This line originates from myocardial tumors induced in 3 day-old quail embryos by microinjection of MC29 avian retrovirus carrying the myc-oncogene. These cells progressively lose their muscle markers with time in culture although co-culture of these cells with 3T3 mouse fibroblasts induces a moderate re-expression of muscle myosin. However, the localization of this protein is diffuse, and the cells did not form an organized contractile cytoskeleton and never exhibited contractile activity. Similar results on the establishment of a cell line from precardiac splanchnic mesoderm of a Japanese quail embryo was recently reported [Eisenberg, Anat. Rec. 232:30A (1992)]. During the establishment of this cell line muscle-specific markers were also lost. They could be induced to express desmin, sarcomeric myosin heavy chain and muscle actin by altering the culture conditions. Neither myofibrils nor contractile activity was detected in these cultured cells. Recently it was reported that human fetal cardiac muscle cells transfected with SV 40 T antigen could be maintained for over a year in culture. [Wang, et al. In Vitro Cell. Dev. Biol. 27:63-74 (1991) ]. These cells expressed several markets of early fetal human cardiac myocytes but did not retain contractile activity. Embryonic rat cardiac myocytes transfected with the v-myc and v-H-ras oncogenes have been reported to undergo multiple passages in culture and to retain the expression of several cardiac-specific genes [Engelmann, et. al. J. Mol. Cell. Cardiol. 25:197-213 (1993)]. In summary, to date no cardiac muscle cell line has been reported or is available that: 1) retains a highly differentiated adult cardiac myocyte phenotype; 2) maintains contractile activity; and 3) can be serially passaged in culture.